Free (or low-cost) tools to create GUI mockups?

Up until now I have been drawing out my mockups on paper with pencil, it works for the most part, but i was wondering if there were professional tools to do this? say for team projects, how are GUI mock up ideas passed around, surely not my neanderthal method of paper and pencil? focusing on desktop applications, I would like some advice. I have also used some of the IDE designers, the most useful and intuitive one I've used thus far is Nokias Qt Designer, because it integrates so well with Qt's MVC pattern and the ability to add slots/signal relationships in the design.

Is there a tool much like Qt Designer but more generalized (and isolated) for just mocking up some UI ideas?

There is a pretty exhaustive list of both commercial and non commercial tools here: http://c2.com/cgi/wiki?GuiPrototypingTools

QT designer is indeed the "closest" IDE for creating mock-ups, but specialized tools are

1. much faster, since they are built for this exact purpose
2. don't require programming knowledge so can be used by analysts and other non-developers
3. usually come with "black&white" or "hand-drawn" skin to avoid getting bogged-down into endless discussions about colors, fonts, etc
4. allowing you to annotate your mock-ups, either to provoke discussion or to use those annotations for specification

(Just to be straight here, I'm the author of such tool - http://MockupScreens.com)